Feed-throughs are well known in the industry and are commonly used to provide electrically conductive paths through the walls of various containers. In many instances, unwanted high-frequency signals generated inside or outside of the container pass through the feed-through and cause problems for circuitry outside or inside the container, respectively.
In an effort to alleviate interference from such sources, various filtering devices and/or techniques have been employed, such as ceramic capacitors. Feed-throughs with capacitors are intended primarily to pass relatively low frequency electrical signals via a lead or conductor through an opening in a conducting wall, screen or the like while presenting a low impedance shunt path to the conducting wall for high frequency signals. Capacitors of this type are disclosed in U.S. Pat. Nos. 3,243,671 and 2,756,375.
This type of feed-through arrangement is commonly used in electronic systems such as heart pacemakers, missiles, satellites and space probes. Illustrative are the feed-through capacitor systems disclosed in U.S. Pat. Nos. 5,032,692, 4,424,551, 4,314,213 and 4,148,003.
To maximize the effectiveness of such filter systems, the feed-through capacitor must be mounted as close to the feed-through as possible. The outside surface of the capacitor is typically covered by a metallic electrode that is connected to the container wall. By positioning the capacitor adjacent to the feed-through, the capacitor becomes a continuation of the container wall. In effect, the container wall "covers" most of the insulator when the capacitor is so positioned. If the high-frequency filter is mounted on a circuit board in the container, the lead from the filter to the feed through can act as an antenna which collects and transmits the high frequency electromagnetic radiation present in the container. Hence, the length of unshielded conductor from the final filter to the wall of the container must be minimized. One method for accomplishing this goal makes use of a feed-through capacitor mounted on the feed-through.
The above described system requires that the capacitor be attached to the feed-through. One method for providing this attachment relies on bonding the capacitor to the feed-through using epoxy. This approach has a number of drawbacks. First, epoxies are difficult to apply, particularly on small packages. This results in higher labor costs and lower yields. Second, epoxies are unstable at temperatures above 175.degree. C. Such temperatures are often encountered when the feed-through assembly is attached to the container via welding or brazing. Third, it is difficult to inspect the assembled part.
One alternative to epoxy attachment is disclosed in U.S. Pat. No. 5,032,692. In this system the capacitor attachment is achieved through the use of brazing alloy rings. The capacitor is brazed into a shielded compartment in the feed-through. While this method eliminates the use of epoxies, it requires capacitors that are capable of withstanding very high temperatures. The use of such capacitors increases the cost of the feed through system.
Broadly, it is the object of the present invention to provide an improved feed-through system.
It is another object of the present invention to provide an electronic feed-through/filter system in which the electronic components mounted on the feed-through are protected from heat transfer during subsequent processing.
It is a further object of the present invention to provide an electronic feed-through filter system which can be more easily inspected after assembly.
Other objects of the invention will become apparent to those skilled in the art from the following description and accompanying drawings.